Organic electroluminescent devices are self-luminous devices based on the phenomenon that electrons combine with holes in a fluorescent or phosphorescent organic layer when a current is applied to the organic layer to generate light. Such organic electroluminescent devices have the advantages of light weight, a small number of components and relatively simple fabrication processes.
Particularly, white organic electroluminescent devices can find applications as light sources for a variety of electric devices, such as liquid crystal displays, and backlight units. White organic electroluminescent devices can also be combined with other elements, such as color filters, in the manufacture of full-color displays.
However, conventional white organic electroluminescent devices have failed to provide satisfactory results in terms of electrical properties, including luminance, current density, power efficiency and chromaticity coordinates.
FIG. 1 is a diagram illustrating the mechanism of light emission from a conventional organic electroluminescent device. As illustrated in FIG. 1, holes are injected into the valance band or the highest-occupied molecular orbital (HOMO) of a material for a hole injection layer (HIL) and enter a light-emitting layer through a hole-transporting layer (HTL), and at the same time, electrons migrate from a cathode to the light-emitting layer through an electron injection layer. The holes combine with the electrons in the light-emitting layer to form excitons, after which the excitons fall to the ground state to emit light.
In the energy transfer process, singlet excitons and triplet excitons are involved in fluorescence and phosphorescence emission, respectively. Therefore, fluorescence can contribute to a maximum of 25% and phosphorescence can contribute to a maximum of 75% in energy efficiency. However, it is known that either singlet or triplet excitons can emit light and both singlet and triplet excitons cannot participate in energy transfer.
An organic electroluminescent device with improved energy efficiency was reported wherein a plurality of light-emitting layers contain the same host and are divided into individual layers emitting fluorescence or phosphorescence (Nature vol. 440, 908 (13, Apr. 2006)).
The structure of the conventional device is shown in FIG. 2. Specifically, the device comprises an anode 110, which is made of indium tin oxide (ITO) by vacuum deposition, a hole injection layer 120, a hole-transporting layer 130, a multilayer structure of a fluorescent layer 140, a green phosphorescent layer 150 and a red phosphorescent layer 160 as light-emitting layers, each of which emits monochromatic light, a hole-blocking layer 170, an electron-transporting layer 180, an electron injection layer 190 and a cathode 200 formed in this order on a transparent substrate 100 by vacuum deposition wherein different colors of light from the light-emitting layers are mixed together to emit polychromatic light.
The organic electroluminescent device is configured to emit fluorescence and phosphorescence from the multilayer structure of light-emitting layers rather than from a single light-emitting layer. In the organic electroluminescent device, electrons are injected from the cathode 200 to the light-emitting layers 140, 150 and 160 through the electron-transporting layer 180, and holes are injected from the anode 110 to the light-emitting layers 140, 150 and 160 through the hole-transporting layer 130. The electrons combine with the holes in the light-emitting layers to form electron-hole pairs (i.e. excitons). The excitons recombine in the respective light-emitting layers containing the same host to allow a fluorescent dopant of the fluorescent layer 140 to emit blue light, a green phosphorescent dopant of the green phosphorescent layer 150 to emit green light and a red phosphorescent dopant of the red phosphorescent layer 160 to emit red light. The different colors of light from the light-emitting layers 140, 150 and 160 are mixed to emit white light. However, the presence of the three light-emitting layers renders the structure of the organic electroluminescent device complex.
Further, an attempt has been made to achieve white light emission using complementary colors (SID 2006, NOVALED). This attempt, however, has many limitations in that the limited complementary color relationship causes poor luminescence efficiency and insufficient intensity of light in a particular wavelength region results in poor color reproducibility.
Thus, there is a need to develop organic electroluminescent devices that have a simple structure, exhibit high luminescence efficiency and have high color reproducibility. There is also a need to develop white organic electroluminescent devices that can maintain their chromaticity coordinates constant irrespective of variations in operating voltage and/or current density.